This technology relates to a device for producing light in a variety of colors. The device incorporates light-emitting diodes and optical frequency converters. The technology has particular, but not exclusive, application in display technology, as a single pixel capable of displaying monochromatic light at a selectable wavelength.
The infrared light emitting diode (LED) was invented by Robert Biard and Gary Pittman in 1961. The visible (red) LED was invented by Nick Holonyak in 1962. LEDs of all colors have been employed in a wide variety of applications for decades, are well understood and documented in the prior art, and need no further elaboration here, except to note that the light emitted by LEDs is generally monochromatic, i.e., it is emitted in a narrow range of wavelengths.
Bi-color and tri-color LEDs also exist, but these are simply multiple monochromatic LEDs arranged in a vertical stack. For example, an LED with independently switchable red and green components can appear not only red or green, but also “orange” when both the red and green components are activated. In this case, the orange color is an illusion resulting from the combination of red and green light. Similarly, full-color LED displays rely on pixels consisting of three different color LED's (usually red, green, and blue, although other combinations are also known to work) which, by being activated with different intensities, produce the illusion of other colors in the human visual range.
The conversion between different wavelengths of light is also well understood in the prior art, and can be accomplished, for example, through the fluorescence of many manmade or naturally occurring materials. Semiconductor materials in particular are understood to fluoresce at a wavelength determined by their bandgap energy, producing a strong, narrow emission peak. Other materials, including phosphorus, fluoresce across many wavelengths. Some colored LEDs are constructed by placing an optical converter in the path of the LED's light output so that, for example, blue light is absorbed and re-radiated as white light (or “approximately white”) when it strikes a converter made of phosphorus.
The fabrication of very small structures to exploit the quantum mechanical behavior of charge carriers, e.g., electrons or electron “holes” is also well established. Quantum confinement of a carrier can be accomplished by a structure whose dimension is less than the quantum mechanical wavelength of the carrier. Confinement in a single dimension produces a “quantum well,” and confinement in two dimensions produces a “quantum wire.”
A “quantum dot” is a structure capable of confining carriers in all three dimensions. Quantum dots can be formed as particles, with a dimension in all three directions of less than the de Broglie wavelength of a charge carrier. Quantum confinement effects may also be observed in particles of dimensions less than the electron-hole Bohr diameter, the carrier inelastic mean free path, and the ionization diameter, i.e., the diameter at which the charge carrier's quantum confinement energy is equal to its thermal-kinetic energy. It is postulated that the strongest confinement may be observed when all of these criteria are met simultaneously. Such particles may be composed of semiconductor materials (for example, Si, GaAs, AlGaAs, InGaAs, InAlAs, InAs, and other materials) or of metals, and may or may not possess an insulative coating. Such particles are referred to in this document as “quantum dot particles.”
A quantum light emitting diode (QLED) is constructed by placing, in the output path of an LED, an optical converter incorporating quantum dot particles. Because the effective bandgap of a quantum dot particle is equal to the material bandgap plus the quantum confinement energy, and because the quantum confinement energy is a function of the size, shape, and composition of the quantum dot particles, it is possible to adjust the converter to fluoresce at nearly any wavelength of visible or infrared light by adjusting these properties at the time of manufacture, or to fluoresce at a variety of wavelengths simultaneously, by incorporating quantum dots of multiple types.
However, a quantum dot can also be formed inside a semiconductor substrate through electrostatic confinement of the charge carriers. This is accomplished through the use of microelectronic devices of various design, e.g., an enclosed or nearly enclosed gate electrode formed on top of a quantum well. Here, the term “micro” means “very small” and usually expresses a dimension of or less than the order of microns (thousandths of a millimeter). The term “quantum dot device” refers to any apparatus capable of generating a quantum dot in this manner. The generic term “quantum dot,” abbreviated “QD” in certain of the drawings herein, refers to the confinement region of any quantum dot particle or quantum dot device.
The optical properties of a material depend on the structure and excitation level of the electron clouds surrounding its atoms and molecules. Quantum dots can have a greatly modified electronic structure from the corresponding bulk material, and therefore different properties. Because of their unique properties, quantum dots are used in a variety of electronic, optical, and electro-optical devices. Quantum dots are currently used as near-monochromatic fluorescent light sources, laser light sources, light detectors including infra-red detectors, and highly miniaturized transistors, including single-electron transistors. They can also serve as a useful laboratory for exploring the quantum mechanical behavior of confined carriers. Many researchers are exploring the use of quantum dots in artificial materials, and as dopants to affect the optical and electrical properties of semiconductor materials.
The embedding of metal and semiconductor nanoparticles inside bulk materials (e.g., the lead particles in leaded crystal) has occurred for centuries. However, an understanding of the physics of these materials has only been achieved comparatively recently. These nanoparticles are quantum dots with characteristics determined by their size and composition. These nanoparticles serve as dopants for the material in which they are embedded to alter selected optical or electrical properties. The “artificial atoms” represented by these quantum dots have properties which differ in useful ways from those of natural atoms. However, it must be noted that the doping characteristics of these quantum dots are fixed at the time of manufacture and cannot be adjusted thereafter.
A single-electron transistor (SET) is a type of switch which relies on quantum confinement. The SET comprises a source (input) path leading to a quantum dot particle or quantum dot device and a drain (output) path exiting, with a gate electrode controlling the dot. With the passage of one electron through the gate path into the device, the switch converts from a conducting or closed state to a nonconducting or open state, or vice-versa.
Semiconductors are capable of serving in optical converters in several ways. The emission wavelength of a fluorescent semiconductor is a function of its bandgap—a material-specific quantity. For photons with energies below the bandgap, the semiconductor is generally transparent, although material-specific absorption bands may also exist. Photons with energies higher than the bandgap absorbed and create electron-hole pairs within the semiconductor. Thus, a material like gallium arsenide (bandgap 1.424 eV) will fluoresce at a wavelength of 871 nanometers.
However, the energy of an electron confined in a quantum well is not only a function of bandgap, but of the quantum confinement energy, which depends on the thickness of the well and the energy height of the surrounding barriers (i.e., the difference in conduction band energy between the well and barrier materials). This “bandgap plus quantum confinement” energy moves the transparency of the material into shorter wavelengths. Thus, while a bulk GaAs sample fluoresces at approximately 870 nm, a 10 nm GaAs quantum well surrounded by Al0.4Ga0.6As barriers has a 34 meV quantum confinement energy and thus shows the same emission peak at approximately 850 nm. Therefore, for a given set of materials and a given reference temperature, the cutoff energy can be fixed precisely through the fabrication of a quantum well of known thickness. It should be noted, however, that the bandgap is a temperature-dependent quantity. As the temperature of a semiconductor decreases, its bandgap increases slightly, and its emission wavelength decreases. When the semiconductor is heated, the bandgap decreases and the emission wavelength increases.
The information included in this Background section of the specification, including any references cited herein and any description or discussion thereof, is included for technical reference purposes only and is not to be regarded as subject matter by which the scope of the invention is to be bound.